The central theme of the project is the clinical assessment of organ perfusion and oxygenation with minimally invasive techniques. The goal of the project is to continue development of a clinically functional system that will permit us to identify and continuously characterize aspects of the cardio-respiratory, muscular, and central nervous system in surgical patients. The characterization utilizes (a) derived variables reflecting basic physiological relationships between the measured variables, and (b) the dynamic response of measured and derived variables to external challenge. The team of physicians and engineers engaged in this project works under a carefully coordinated approach in which clinical work in the operating room is supported by animal experiments, mathematical modeling, instrument development, and a computer system. The specific areas under study involve assessment of the heart by measuring external cardiac indices and estimating cardiac output. In addition, parasympathetic influences on the heart will be evaluated by analysing heart rate variability. The blood supply to tissue compartments is assessed on the basis of wash-in and wash-out functions of tracer tissue gases. Studies of the lung utilize a number of traditional and new procedures. Among the latter is an effort to obtain information on intrathoracic pressures from central venous pressures. Analysis of rapid changes in respiratory gas tensions occurring with changes in breathing pattern are taken to indicate lung ventilation and perfusion. Muscle function is studied using electrical stimulation and recording of muscle potentials. CNS function is studied by measurements of brain pressure and recording of cortical evoked potentials. Physiologic data from the O.R. of a remote hospital will be transmitted to the parent facility for computation and return of the analyzed data.